console.log(k)//报错
console.log(y)// undefined，不会报错
var x = 1
var y = 2
sum(x, y) //不会报错
sum1(x, y)//报错
function sum(a, b) {
    console.log(a+b)
    return a + b;
}

var sum1 = function (a, b) {
    console.log(a+b)
    return a + b
}

